| With the increasing concerns of living environment the environmental safety of wood conposites for interior decoration and furnifure has obtained more and more attentions, and therefore, development of environmentally friendly wood adhesive from bioresources has become an inevitable trend. As a typical abundant and renewable bioresource, Soybean protein is able to prepare new wood adhesive. Due to the complexity quaternary structure and large molecular weight, soybean protein-based adhesive has some fatal disadvantages such as low solid content, high viscosity, poor water resistance and bond strength that prevent it from wide application in the wood industry. In this paper, a modified soybean protein-based adhesive with high solid content, low viscosity, excellent bonding strength, suitable water resistance and good technological applicability was developed using alkali-degraded soybean protein (D-SPI), thermal-acid treated soybean protein(A-SPI) and ployamide-epichlorohydrin (PAE) resin as crosslink. This study mainly focused on the preparation and performance optimization of the crosslinker.The synthetic technology of PAE resin was investigated by two-step process by evaluating the effects of polyamine species, polyamine/adipic acid molar ratio, reaction temperature and time, epichlorohydrin/polyamine molar ratio, alkyation process, and the species of terminating acid and its content on the properties of PAE resin. An optimal synthetic technology was obtained as:the preferable poly amide was prepared with adipic acid and diethylenetriamine at a molar ratio of 1/1.05 and the mixture of charged adipic acid and diethylenetriamine was sufficiently dissolved at 125℃ before heating up 185℃ for 2h. The preferable alkylation for PAE was kept the mixture of polyamide and epichlorohydrin (moleratio of 1:1) at 35℃ for 0.5h before it heated up to 50℃ for reaching target viscosity, and the formic acid and sulfuric acid was used to adjust the pH of final product to 3.5. The optimal PAE resin had advantages of suitable crosslinking degree sufficient epichlorohydrin branches and cation content, longer storage period and higher solid content.The optimal PAE resin was used to modify the soybean protein adhesive and compared with three common modifiers, namely, glyoxal, epoxy resin and polyisocyanate. Results indicated that the PAE resin could effectively improve the bond strength and water resistane of soybean adhesives which were better than the UF resin. The wet strength of plywood after 28h boiling-dry-boiling treatment was up to 1.12MPa, higher than the required value for Type Ⅰ plywood according to Chinese Standard in GB/T17657-1999. This laied the fundaments for replacing formaldehyde-based adhesive in the wood industry by the soybean protein-based adhesive.The formulation and modifying mechanism of PAE-modified soybean protein adhesive were investigated by FT-IR, NMR, TG, SEM and plywood evaluation. Results indicated that the reaction of azetidine with amino, carboxyl and hydroxyl of wood and protein led to the formation of stable networks among PAE, soybean protein and wood, resulting in the good water resistance. The preferable PAE-modified soybean protein adhesive was prepared with D-SPI/A-SPI mass ratio of 1/3 and PAE level of 30% on basis of protein mass, which had the beat bond strength, water resistance and technological applicability. |
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